How to return COM object?

[RYU]

Hi, I tried to return an interface but it gave me this error:

Unable to cast COM object of type 'MyTestLib.MainTestClass' to interface type 'MyTestLib.IMainTest '. This operation failed because the QueryInterface call on the COM component for the interface with IID '{519A413A-2792-4021-847F-B7C205FFE057}' failed due to the following error: No such interface supported (Exception from HRESULT: 0x80004002 (E_NOINTERFACE)).

Any idea why? Here is my IDL:

import "oaidl.idl";
import "ocidl.idl";

[ object, uuid(EC23249E-2891-4f06-9D6E-77895E74448F) ]
interface IAdd : IUnknown
{
    HRESULT     SetFirstNumber(long nX1);
    HRESULT     SetSecondNumber(long nX2);
    HRESULT     DoTheAddition([out,retval] long *pBuffer);
};

[ object, uuid(519A413A-2792-4021-847F-B7C205FFE057) ]
interface IMainTest : IUnknown
{
    HRESULT     GetAddObject([out,retval] IAdd **ppObj);
};

[ uuid(AA484B10-6099-4e37-AEE3-8B4ADFBE2815) ]
library MyTestLib
{
    importlib("stdole32.tlb");

    [ uuid(82B76786-E89B-4e42-8C50-B7916FEC1ADC) ]
    coclass AddClass
    {
        [default] interface IAdd;
    }

    [ uuid(E76E5E58-F9D9-4570-AEC5-79A72E8E8743) ]
    coclass MainTestClass
    {
        [default] interface IMainTest;
    }
};

Thanks for any help :)

[RYU]

I found the problem. Apparently I am trying to be really smart about this by creating only 1 Factory and put a check in CreateInstance() to determine whether it is AddClass or MainTestClass. I was wrong because the IID check should be in DllGetClassObject(). I have created 2 different factories for these classes and it works fine. I can now return the AddClass by using GetAddObject() function. However, when I tried to use the function in AddClass (eg. SetFirstNumber()), it gave me this error:

The runtime has encountered a fatal error. The address of the error was at 0x7f628678, on thread 0x105c. The error code is 0xc0000005. This error may be a bug in the CLR or in the unsafe or non-verifiable portions of user code. Common sources of this bug include user marshaling errors for COM-interop or PInvoke, which may corrupt the stack.

Here is my code:

HRESULT __stdcall CMainTestClass::GetSamTest(IAdd **ppRetVal)
{
    CAddClass *pAddClass = new CAddClass();
    *ppRetVal = pAddClass;
    return S_OK;
}

Thanks again for any help :)